1. Field of the Invention
The present invention relates to a composition for treating surfaces of copper and copper alloys comprising a specific imidazole derivative, and to a method for treating surfaces of copper and copper alloys using this composition. The composition of the present invention is particularly useful as a rust preventing agent for printed-wiring boards.
2. Description of the Background Art
There are two methods for preventing rust in a circuit formed from copper or a copper alloy on a printed-wiring board and for maintaining good solderability. One is a method of covering the circuit with another metal and the other is a method of coating the circuit with organic films. Either of the two methods is used depending on the allowable cost, requirements for surface smoothness, and the like.
Examples of materials which form the organic film used in the last-mentioned method include a rosin-type coating material, which coats the entire printed-wiring board, and an alkylimidazole-type coating material which forms a film by a chemical reaction selectively on the copper circuit section.
The rosin-type coating material is used in a method for forming a film by coating, spraying, or soaking the entire printed-wiring board with a solution of a natural rosin, a rosin ester, a rosin-modified maleic acid resin, or the like, in an organic solvent, and then drying. However, this method involves a problem of impairing the working environment and the safety because of volatilization of the organic solvent. The treating plant must be provided with special equipment such as draft for air discharge.
Therefore, there have been desires for the use of water-soluble alkylimidazole-type coating materials which are excellent materials from the aspect of keeping a good working environment and safety. Films made from alkylimidazole-type coating materials, however, deteriorate when subjected to a high temperature, hindering the action of a postflux which is used during soldering and thus giving a rise to the drawback of poor solderability.
In recent years, a surface mounting method has become the most commonly used method for joining electronic parts on printed-wiring boards. This increases opportunities wherein printed-wiring boards are exposed to high temperatures, e.g., temporary mounting of the parts, reflow of solder pastes, and the like. For this reason, a water-soluble type surface treatment agent which has high heat resistance and exhibits excellent solderability, even when the printed-wiring boards have been exposed to high temperatures, is required.
In order to satisfy this requirement, Japanese Patent Laid-open (ko-kai) No. 124395/1991 discloses a coating agent comprising a benzimidazole derivative with a hydrogen atom, an alkyl group, or a phenyl group in 2-position of the imidazole ring.
In spite of the attempt of this laid-open patent to improve the heat resistance of alkyl imidazole type coating agents, the problems have not been resolved to the satisfaction of the industry.
Therefore, there have been a strong demands for a composition for treating surfaces of copper and copper alloys which exhibits improved heat resistance and provides a good working environment and safety, as well as for a method of the surface treatment.
In view of this situation, the present inventors have undertaken extensive studies in order to develop a water-soluble surface treating agent which does not require the use of volatile solvents and exhibits excellent solderability even after exposed a high temperature, and found that specific types of benzimidazole derivatives, represented by formulas (1) to (6) below or salts thereof, can produce a film which is heat resistant and exhibits excellent solderability. This finding has led to completion of the present invention.